1. Field of the Invention
The present invention relates to a toner used for electrophotographies, electrostatic recording methods, electrophotographic printings, etc., and more particularly to a color toner including a titania as an additive and a release agent, which has good transparency, color reproducibility, fluidity, fixability, offset resistance, and durability without filming, and which can be used for a fixer that does not use an oil in its fixing member. In addition, the present invention relates to a method for manufacturing the toner, and to an image forming apparatus and method using the toner.
2. Discussion of the Background
As background electrophotographic methods, various methods are disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications Nos. 42-23910 and 43-24748, etc. Typically, in such methods an electrostatic latent image is formed on a photoconductive substrate using a photoconductive material and the latent image is developed with a toner to visualize the image, or optionally the toner image is transferred onto a paper, etc. and fixed by heating, pressurizing, or solvent steaming to form the visual image. As a full-color electrophotographic method to form a multi-color image, U.S. Pat. No. 2,962,374 discloses a method in which an image is irradiated with at least blue, green, and red color signals and the resultant latent image is developed repeatedly with a process color toner such as at least a yellow, a magenta, a cyan toner, etc. to multiply each color toner image to form a multi-color image.
Recently, hard copy technologies using the electrophotographic methods are rapidly developing into full-color copy technologies from mono-color technologies, and the full-color copy market is quickly expanding. The full-color electrophotographic methods typically reproduce all colors using three primary color toners, i.e., a yellow, a magenta, and a cyan color toner, or four colors including a black color toner. The typical method includes steps (1)-(3) as follows:    (1) a latent image is formed on a photoconductive layer of the photoconductive substrate with light from an original through a color-separation optical-transmission filter having a complementary color with the toner color;    (2) the latent image is developed with the toner and transferred onto a receiving material on which the toner is retained; and    (3) the above-mentioned processes are repeated, and the toners are multiplied on the same receiving material while the registration is adjusted to form the final full-color image by only one-time fixation.
In such a full-color electrophotographic method that performs plural development and multiplies different color toner images on the same receiving material, the fixability of the color toner is quite an important factor. Namely, the fixed color toner needs to prevent the irregular reflections by the toner particles as much as possible and needs adequate glossiness. In addition, the color toner needs to have transparency not to prevent seeing the color tone of the toner in the underlayer of the toner layers, and good color reproducibility.
Typically, the color toner is manufactured by kneading a binder resin and a colorant or optionally with other materials such as a charge controlling agent upon application of heat, pulverizing the cooled material, and classifying the pulverized material into particles having specified particle sizes.
As the colorant used for the color toner, organic pigment colorants having better light resistance and safety than dye colorants are typically used.
However, the organic pigment colorant forms agglomerated primary particles of the organic pigment in a drying process of manufacturing the pigment and the above-mentioned background color toner manufacturing method cannot dissolve the agglomeration. Therefore, the colorant is difficult to disperse in the binder resin and the agglomerated primary particles of the pigment remains in the toner. Accordingly, the transparency, the color reproducibility, the optical permeability in an OHP film, and the colorability of the resultant color toner largely deteriorate, and particularly this tendency remarkably appears in a color toner having a small particle diameter.
In order to solve this problem, Japanese Laid-Open Patent Publications Nos. 62-30259, 62-280755, 266561, 2-293866, 2-293867, etc. disclose a method in which the dispersion of an organic pigment in a toner is improved by using a modified pigment ordinarily called a master batch. The modified pigment is manufactured by kneading a dry organic pigment with a binder resin upon application of heat such that the concentration of the pigment is higher than that of an ordinary pigment used for a toner, and kneading the kneaded material further while diluting the kneaded material with a binder resin such that the resultant toner has an adequate pigment concentration. However, the agglomerated particles are still difficult to finely disperse until they do not affect the optical permeability of the toner even by using the above-mentioned method.
As the other method, a method in which the master batch is formed by dissolving and mixing the materials in, for example, a solvent and evaporating the solvent is disclosed in Japanese Laid-Open Patent Publications Nos. 61-117565 and 61-156054, etc. However, only agitating a liquid-solution mixture cannot sufficiently disperse a colorant due to insufficient shearing force.
In addition, Japanese Laid-Open Patent Publication No. 62-127847 discloses a method in which a dispersion liquid of fine particles of a phthalocyanine pigment prepared by an acid paste method or an acid slurry method is mixed in an organic liquid solvent with a binder resin without drying the dispersion liquid to uniformly disperse the fine particles of the phthalocyanine pigment in the binder resin. However, the method has a problem that a large quantity of the organic solvent is needed to dissolve the binder resin and that the pigment tends to reaggregate when removing the water and organic solvent.
Japanese Laid-Open Patent Publication No. 7-311479 discloses a method in which a pigment-dispersion resin has a different solubility parameter from that of a binder resin and cannot be mixed therewith, and which is prepared by mixing a paste of a pigment including in water in a liquid-solution of a resin for dispersing the pigment and heating the mixture to form a toner having good transparency, offset resistance, and preventing an intertwist of a receiving paper. However, since the binder resin and the resin to disperse the pigment have different solubilities from each other, the resultant toner has a high refraction index and does not have sufficient transparency.
On the other hand, an additive such as a silica is typically included in a toner to improve its properties such as fluidity, transferability, and developability. When the content of the additive is small, the fluidity of the toner deteriorates and the toner tends to adhere to each other, resulting in the occurrence of toner filming on a photoreceptor, image-transfer irregularity and white spots on a solid image, a hollow defect on a thin line image, background fouling, and an increase of toner scattering. Therefore, the content of the additive has to be increased, and particularly the content thereof has to be increased further for a toner having a small particle diameter to keep the coverage because the specific surface area of such a toner increases.
In order to decrease the mutual toner adherence causing the image-transfer irregularity and white spots in a solid image, and a hollow defect in a thin line image, increasing the molecular weight of the binder resin can be considered. However, the color toner typically needs more fixing heat than the black toner to melt and to have low viscosity for having glossiness and transparency, but resin having a large molecular weight has a high softening temperature and the viscosity does not decrease even when the fixing temperature is high. Therefore, the resultant toner does not have sufficient glossiness and transparency. In addition, an excessive addition quantity of the additive causes a toner-scattered image in a thin line image, and particularly when a thin line image is produced in a full-color image, the toner quantity increases because at least two or more color toners have to be multiplied and the tendency remarkably appears.
When only the silica is added to a toner, the chargeability of the toner largely depends on the environment and the charge of the toner increases because the silica itself has high charging capability. Particularly, toner having a small particle diameter tends to have an excessive charge due to the mutual friction of the toner because of its large specific surface area. In order to solve this problem, a surface-treated silica or a titania instead of the silica can prevent the excessive charge due to the friction and the dependence of the toner chargeability on the environment to maintain an adequate charge. However, the surface-treated silica cannot by itself sufficiently prevent the dependence of the toner chargeability on the environment, while the titania effectively prevents the dependence of the toner chargeability on the environment, but deteriorates the transparency and color reproducibility of the resultant toner. Particularly, the titania is typically added to a toner more than an ordinary silica because it has a larger specific gravity than the silica, and therefore the transparency, color reproducibility, toner scattering, and fixability of the resultant toner deteriorate further. Particularly, for color toner that has to be stable for a long time and to produce an image having high image quality and color reproducibility, the deterioration of the transparency and color reproducibility of the toner is not preferable because that is one of the largest factors in deteriorating the image quality.
On the other hand, although a fixer fixing the color toner has a member made of a material having good surface releasability, the surfaces of most members are coated with an oil. Coating a large quantity of the oil on the fixing member to increase the releasability causes an oil spot on a receiving paper and a cost increase. In addition, a space for a tank containing the oil is needed, resulting in a problem that the fixer is enlarged.
The reason why the oil is typically coated on the fixing member is as follows. The color toner typically needs more fixing heat than the black toner to melt and to have low viscosity for having glossiness and transparency. However, a toner having such a resin tends to decrease the intermolecular agglomeration when melted upon application of heat and to adhere to the fixing member when passing the member, resulting in occurrence of a high-temperature offset phenomenon. Therefore, in order to prevent the high-temperature offset phenomenon, an oil is typically coated on the fixing member to decrease the toner adherence thereto.
On the other hand, using an oilless toner is attempted without coating the oil on the fixing member. A method in which a release agent is dispersed in a toner is disclosed in Japanese Laid-Open Patent Publications Nos. 8-220808, 9-106105, 9-304964, 9-304971, 10-2071126, 10-207126,10-254173, 10-293425, 11-2917, 11-24313, 11-249341, etc.
However, as for the color toner in which the release agent is dispersed, the toner does not have sufficient transparency because the release agent is present on the surface of the toner, and the fluidity of the toner also largely deteriorates. Further, the release agent has to sufficiently filter from the color toner having a low viscosity and the offset prevention is difficult. To the contrary, a toner having high viscosity such as a black toner has a high intermolecular agglomeration when melted upon application of heat and the offset can be prevented if only a small quantity of the release agent filters from the toner, but the toner is not sufficiently melted and does not have sufficient transparency, color reproducibility, and glossiness.
Because of the above-mentioned reasons, a color toner including a titania and a release agent, and having the following properties (1)-(4), has not been realized:    (1) good transparency, color reproducibility, fixability, and offset resistance;    (2) applicability to a fixer without using an oil on the fixing member;    (3) freedom from background fouling, toner scattering, toner filming, and image defects such as hollow defects, image-transfer irregularity and white spots; and    (4) capablity of producing images having stable image quality for a long time.
On the other hand, a cleaner is conventionally used to remove a residual toner from an image bearer, i.e., a photoreceptor, and to collect the toner. Further, many technologies to recycle the collected toner by the cleaner for development are suggested. In addition, recently a belt is used as a transferer, and it is also suggested that the residual toner on the belt is removed and recycled.
However, there are some problems to be considered when recycling the residual toner on the photoreceptor and the transferer for development. For example, when removing the residual toner from the photoreceptor, a blade is typically used to scrape the toner. On an area of the photoreceptor contacting the blade, the agglomerated toner due to the mutual adherence of the toner and fine particles of the toner due to the pulverization thereof tend to be formed, resulting in deterioration of the toner properties. Consequently, when such a toner is recycled for development, the deteriorated toner such as the agglomerated toner and the pulverized fine particles thereof occasionally causes an abnormal image such as a decrease of the image density, background fouling, and toner scattering.
This is considered to be caused by the toner tending to adhere to each other because the additive such as silica and titania to increase the surface fluidity of the toner is not uniformly adhered to the surface of the impacted toner, or the additive is released from or buried in the surface thereof. When such a toner is used for development, the toner having a large particle diameter adheres to the photoreceptor. Therefore, image-transfer irregularity and white spots on a solid and a halftone image, image-density deterioration, background fouling, and toner scattering occur, and images having stable image quality cannot be produced over a long time.
Japanese Patent No. 2636300 discloses a toner including a specific polyester including fatty acid diol as a binder resin used for a recyclable image forming method. However, the toner particles are easily broken by a mechanical force such as friction and agitation when transferred to the cleaning and developing portion because polyester including aliphatic diol is more flexible than polyester including aromatic diol and the glass-transition temperature tends to decrease. Therefore, the fine particles of the toner tend to be formed, causing background fouling of the resultant image, contamination inside the image forming apparatus by the toner, and deterioration of the developability. Further, the mutual toner adherence increases and tends to be agglomerated because the additive such as silica and titania is easily buried in the toner. In addition, the toner filming on the photoreceptor occurs and the fluidity and heat resistance of the toner deteriorate. Particularly, the tendency remarkably appears under a condition of high temperature and high humidity.
Japanese Laid-Open Patent Publication No. 8-234483 discloses a toner including polyester having specific properties as a binder resin, and inorganic fine particles having a specific quantity of free agglomerated particles and a volume-average primary particle diameter of from 5 to 50 nm. The quantity of the free agglomerated particles of the inorganic fine particles is defined as (1)-(3) as follows:    (1) the free agglomerated particles having a particle diameter not less than 47 μm are not greater than 0.05% by weight per 100% by weight of the toner;    (2) the free agglomerated particles having a particle diameter not less than 20 μm and less than 47μm are 0.05% by weight per 100% by weight of the toner; and    (3) the free agglomerated particles having a particle diameter not less than 2 μm and less than 20 μm are not greater than 1.0% by number per 100% by number of the toner
However, the free inorganic fine particles cause the toner filming on a photoreceptor. The agglomerated fine particles scratch the photoreceptor and the inorganic fine particles accumulate in the scratch to cause image defects such as white spots. In addition, image density deteriorates, and background fouling and toner scattering increase as time passes. Therefore, images having stable image quality cannot be produced over a long time.
Japanese Laid-open Patent publication No. 8-15908 discloses that a toner including specific polyester as a binder resin, to which inorganic fine particles having an average particle diameter not less than 30 nm and less than 100 nm are adhered, is used for a recyclable image forming method. The inorganic fine particles having a larger particle diameter than that of ordinary inorganic fine particles effectively prevent the inorganic fine particles from being buried in the surface of the toner. However, depending on the mixed and adhered conditions of an additive, the inorganic fine particles releasing from the toner increase and cause the toner filming on the photoreceptor and the developing sleeve. In addition, the toner from which the inorganic fine particles are releasing decreases the quantity of the adhered fine particles as time passes, and the toner particles adhere to each other. Therefore, agglomerated particles are formed when the toner is transferred to the photoreceptor. The inorganic fine particles having a larger particle diameter than that of the ordinary fine particles tend to scratch the photoreceptor, and the fine particles accumulate in the scratch causing image defects such as white spots.
In addition, other prior technologies are disclosed in Japanese Laid-Open Patent Publications Nos. 62-195677, 7-92726, 7-128902, 7-295242, 7-319208, 8-30014, 10-254173, etc.
Further, an image forming apparatus and method in which a toner image formed on a photoreceptor is transferred onto a receiving material such as paper through an intermediate transfer medium such as an intermediate transfer belt are known. Such an image forming apparatus and method are widely used in apparatuses forming color images. For example, four color toner images, i.e., a black (Bk), a yellow (Y), a magenta (M), and a cyan (C) color toner image, are transferred and multiplied sequentially on the intermediate transfer medium at first. The multiplied toner images of Bk, Y, M, and C are transferred from the intermediate transfer medium onto the receiving material at a time and fixed thereon to form a color image. Compared with a method in which a toner image is directly transferred onto a receiving material, the intermediate transfer medium has an advantage to improve problems such as color drifts when the toner images are multiplied and faulty transfers due to the difference of the receiving materials. This is an important reason why the intermediate transfer medium is used when forming a color image.
In addition, recently, a tandem type image forming apparatus and method are known, in which plural photoreceptors are arranged in parallel and an image developer is arranged for each photoreceptor. A singlecolor toner image is formed on each photoreceptor and each single-color toner image is sequentially transferred onto a receiving material such as paper to form a color image. Further, an image forming apparatus and method combining this tandem type method and the above-mentioned intermediate transfer method are used.
Particularly, in the image forming apparatus and method using the intermediate transfer medium, a roller transferer through a roller is typically used for the first transfer from the photoreceptor to the intermediate transfer medium and the second transfer therefrom to the receiving material. When the transfer is performed, the toner image is pressurized by the roller and the adherence of the toner to the photoreceptor, intermediate transfer medium, or the receiving material increases. In addition, the agglomeration of the toner increases the mutual toner adherence and a local point of the toner is strongly pressurized, and therefore the point is not transferred to cause hollow defects of the resultant image. Further, the toner filming on the photoreceptor or the intermediate transfer medium, particularly the hollow defects, are remarkably formed on the thin line images.
As the first transfer method from the photoreceptor to the intermediate transfer medium, an indirect pressing method in which the roller is arranged downstream of the photoreceptor and a direct pressing method in which the roller directly pressurizes the photoreceptor through the intermediate transfer medium are known. In the indirect pressing method, the roller is arranged downstream of the photoreceptor, and the distance and space between the photoreceptors have to be longer and larger than those of the direct pressing method to avoid the mutual influences of the photoreceptors. Particularly, in the image forming apparatus and method combining the tandem and intermediate transfer methods, a larger space is needed since the plural photoreceptors and image developers are arranged in parallel. Therefore, in order to downsize the apparatus and cut cost thereof, the diameter of the photoreceptor has to be smaller to shorten the distance between the photoreceptors.
Accordingly, the direct pressing method is typically used for the image forming apparatus and method combining the tandem and intermediate transfer methods. However, as mentioned above, the intermediate transfer medium directly pressurizes the photoreceptor in the direct pressing method and the adherence of the toner to the photoreceptor and the intermediate transfer medium increases. In addition, the agglomeration of the toner increases the mutual toner adherence and the toner has a local point strongly pressurized. Therefore, the point is not transferred to cause the hollow defects of the resultant image. Further, the toner filming on the photoreceptor and the intermediate transfer medium tends to occur.